Extending animation clip issue
For this kind of animation (long distance walk) you could use a walk cycle and make the armature follow a curve, but, as ZargulTheWizard says, it is pretty easy to fix in the Graph Editor:

First delete the 3 last keyframes (which are identical), select the whole series of keyframes, duplicate them whith ShiftD, move them on the right and keep them selected:



Switch to Graph Editor with shortcut CtrltTab, unfold the Armature's tracks on the left, select the track called Z Location (mixamorig.Hips) which is the root bone, and make it the only one visible with a ShiftH:



Now move up the handles until it fits with the beginning of the curve (keep pressing Shiftt if you want to move more precisely). Now the character will continue its walk as it should: